1. Field
Embodiments of the disclosure relate generally to the field of tailoring of surface geometries for aerodynamic improvements to aircraft or surfaces having a flow interface and more particularly to embodiments and fabrication methods for use of both very stiff materials (such as nickel) and materials with a significant, but recoverable, elongation (such as high elongation elastomeric polymers and shape memory polymers and metals) to form aerodynamic riblets or other high-aspect-ratio surface microstructures requiring high durability.
2. Background
Increasing fuel efficiency in modern aircraft is being accomplished through improvement in aerodynamic performance and reduction of weight for both structural and non-structural components. Recent advances in the use of microstructures such as riblets on aerodynamic surfaces have shown significant promise in reducing drag to assist in reducing fuel usage. Riblets have various forms but advantageous embodiments may be ridge-like structures that minimize drag on the surface of an aircraft. Riblets may be used in areas of a surface of an aircraft exposed to a turbulent boundary layer. The riblet ridges tend to inhibit turbulent motions involving lateral velocities, thus reducing the intensity of small-scale streamwise vortices in the lower part of the boundary layer, and thus reducing skin-friction drag.
In certain tested applications riblets have been pyramidal or inverted V shaped ridges spaced on the aerodynamic surface to extend along the surface in the direction of fluid flow. Riblet structures have typically employed polymeric materials, non-elastomeric thermoplastic or thermoset polymers. However in service use such as on an aircraft aerodynamic surface, these polymer riblets are relatively easily damaged thus reducing the performance of the riblet and degrading the appearance of the surface. These surfaces were readily gouged by tools, edges of boots, maintenance equipment impacting or rubbing along the surface resulting in the need to replace or remove the riblets. This lack of durability has been the key impediment to the use of riblets for drag reduction. Non-elastomeric polymeric riblets may readily fracture or permanently deform hundreds of percent with tool- or fingernail-induced pressure. Thermoplastic polymers (fluoropolymers such as the ter-polymer tetrafluoroethylene hexafluorpropylene vinylidene fluoride (THV), fluorinated ethylene propylene (FEP) or polyethylene for example can undergo large deformations (hundreds of percent elongation) without breaking but those deformations will be largely unrecoverable destroying the both the appearance and the drag reduction benefits of the riblet structure. Thermosetting amorphous polymers (structural epoxies for example) deformed beyond their elastic limit cavitate and crack at low strains (typically at <10% elongation). Non-elastomeric polymers deform readily with a fingernail cross wise to the riblet ridges/grooves, either by plastic deformation or by cavitation and cracking. Such structures may be undesirable in normal service use on an aircraft or other vehicle.
The practicality of riblets for commercial aircraft use would therefore be significantly enhanced with a riblet structure providing increased durability.